choose better index when the index does not need to touch the table data

when we can satisfy a query from an index without touching table data,
then that index should have a lower cost than an index that DOES need
to retrieve table data

h2/src/main/org/h2/index/BaseIndex.java

@@ -5,11 +5,15 @@
  */
 package org.h2.index;
 
+import java.util.Arrays;
+import java.util.HashSet;
+
 import org.h2.api.ErrorCode;
 import org.h2.engine.Constants;
 import org.h2.engine.DbObject;
 import org.h2.engine.Mode;
 import org.h2.engine.Session;
+import org.h2.expression.ExpressionVisitor;
 import org.h2.message.DbException;
 import org.h2.message.Trace;
 import org.h2.result.Row;
@@ -21,6 +25,7 @@ import org.h2.table.IndexColumn;
 import org.h2.table.Table;
 import org.h2.table.TableFilter;
 import org.h2.util.MathUtils;
+import org.h2.util.New;
 import org.h2.util.StatementBuilder;
 import org.h2.util.StringUtils;
 import org.h2.value.Value;
@@ -148,7 +153,7 @@ public abstract class BaseIndex extends SchemaObjectBase implements Index {
      * b-tree range index. This is the estimated cost required to search one
      * row, and then iterate over the given number of rows.
      *
-     * @param masks the search mask
+     * @param masks the IndexCondition search masks, one for each Column in the table
      * @param rowCount the number of rows in the index
      * @param filters all joined table filters
      * @param filter the current table filter index
@@ -233,6 +238,26 @@ public abstract class BaseIndex extends SchemaObjectBase implements Index {
                 cost -= coveringCount;
             }
         }
+        // If we have two indexes with the same cost, and one of the indexes can satisfy the query
+        // without needing to read from the primary table, make that one slightly lower cost
+		HashSet<Column> set1 = New.hashSet();
+        for (int i=0; i < filters.length; i++) {
+        	if (filters[i].getSelect() != null) {
+        		filters[i].getSelect().isEverything(ExpressionVisitor.getColumnsVisitor(set1));
+        	}
+        }
+        if (!set1.isEmpty()) {
+			HashSet<Column> set2 = New.hashSet();
+			for (Column c : set1) {
+				if (c.getTable() == getTable()) {
+					set2.add(c);
+				}
+			}
+	        set2.removeAll(Arrays.asList(getColumns()));
+			if (set2.isEmpty()) {
+				cost -= 1;
+			}
+        }
         return cost;
     }
 

h2/src/main/org/h2/mvstore/db/MVPrimaryIndex.java

@@ -219,8 +219,10 @@ public class MVPrimaryIndex extends BaseIndex {
     public double getCost(Session session, int[] masks,
             TableFilter[] filters, int filter, SortOrder sortOrder) {
         try {
-            long cost = 10 * (dataMap.sizeAsLongMax() + Constants.COST_ROW_OFFSET);
-            return cost;
+        	// we use 9 here and 10 in MVSecondaryIndex to biase the decision towards
+        	// using a table-scan when it has similar cost to an index
+            return 9 * getCostRangeIndex(masks, dataMap.sizeAsLongMax(),
+                    filters, filter, sortOrder);
         } catch (IllegalStateException e) {
             throw DbException.get(ErrorCode.OBJECT_CLOSED, e);
         }

h2/src/main/org/h2/table/Plan.java

@@ -6,7 +6,9 @@
 package org.h2.table;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.HashMap;
+
 import org.h2.engine.Session;
 import org.h2.expression.Expression;
 import org.h2.expression.ExpressionVisitor;
@@ -104,12 +106,17 @@ public class Plan {
      * @return the cost
      */
     public double calculateCost(Session session) {
+    	if (session.getTrace().isDebugEnabled()) {
+    		session.getTrace().debug("Plan       : calculate cost for plan {0}", Arrays.toString(allFilters));
+    	}
         double cost = 1;
         boolean invalidPlan = false;
         for (int i = 0; i < allFilters.length; i++) {
             TableFilter tableFilter = allFilters[i];
+            session.getTrace().debug("Plan       :   for table filter {0}", tableFilter);
             PlanItem item = tableFilter.getBestPlanItem(session, allFilters, i);
             planItems.put(tableFilter, item);
+            session.getTrace().debug("Plan       :   best plan item cost {0} index {1}", (int)item.cost, item.getIndex().getPlanSQL());
             cost += cost * item.cost;
             setEvaluatable(tableFilter, true);
             Expression on = tableFilter.getJoinCondition();
@@ -123,6 +130,7 @@ public class Plan {
         if (invalidPlan) {
             cost = Double.POSITIVE_INFINITY;
         }
+        session.getTrace().debug("Plan       : plan cost {0}", (int)cost);
         for (TableFilter f : allFilters) {
             setEvaluatable(f, false);
         }

h2/src/main/org/h2/table/Table.java

@@ -697,11 +697,13 @@ public abstract class Table extends SchemaObjectBase {
         PlanItem item = new PlanItem();
         item.setIndex(getScanIndex(session));
         item.cost = item.getIndex().getCost(session, null, filters, filter, null);
+        session.getTrace().debug("Table      :     potential plan item cost {0} index {1}", item.cost, item.getIndex().getPlanSQL());
         ArrayList<Index> indexes = getIndexes();
         if (indexes != null && masks != null) {
             for (int i = 1, size = indexes.size(); i < size; i++) {
                 Index index = indexes.get(i);
                 double cost = index.getCost(session, masks, filters, filter, sortOrder);
+                session.getTrace().debug("Table      :     potential plan item cost {0} index {1}", cost, index.getPlanSQL());
                 if (cost < item.cost) {
                     item.cost = cost;
                     item.setIndex(index);

h2/src/test/org/h2/test/db/TestOptimizations.java

@@ -62,6 +62,7 @@ public class TestOptimizations extends TestBase {
         testNestedInSelect();
         testInSelectJoin();
         testMinMaxNullOptimization();
+        testUseCoveringIndex();
         // testUseIndexWhenAllColumnsNotInOrderBy();
         if (config.networked) {
             return;
@@ -1027,4 +1028,21 @@ public class TestOptimizations extends TestBase {
 
         conn.close();
     }
+    
+    private void testUseCoveringIndex() throws SQLException {
+        deleteDb("optimizations");
+        Connection conn = getConnection("optimizations");
+        Statement stat = conn.createStatement();
+        stat.execute("CREATE TABLE TBL_A(id IDENTITY PRIMARY KEY NOT NULL,  name VARCHAR NOT NULL, active BOOLEAN DEFAULT TRUE,  UNIQUE KEY TBL_A_UK (name) )");
+        stat.execute("CREATE TABLE TBL_B(id IDENTITY PRIMARY KEY NOT NULL,  tbl_a_id BIGINT NOT NULL,  createDate TIMESTAMP DEFAULT NOW(),  UNIQUE KEY TBL_B_UK (tbl_a_id, createDate),  FOREIGN KEY (tbl_a_id) REFERENCES TBL_A(id) )");
+        stat.execute("INSERT INTO TBL_A (name)  SELECT 'package_' || CAST(X as VARCHAR) FROM SYSTEM_RANGE(1, 100)  WHERE X <= 100");
+        stat.execute("INSERT INTO TBL_B (tbl_a_id, createDate)  SELECT CASE WHEN tbl_a_id = 0 THEN 1 ELSE tbl_a_id END, createDate  FROM ( SELECT ROUND((RAND() * 100)) AS tbl_a_id, DATEADD('SECOND', X, NOW()) as createDate FROM SYSTEM_RANGE(1, 50000) WHERE X < 50000  )");
+        stat.execute("CREATE INDEX tbl_b_idx ON tbl_b(tbl_a_id, id)");
+	    stat.execute("ANALYZE");
+
+	    ResultSet rs = stat.executeQuery("EXPLAIN ANALYZE SELECT MAX(b.id) as id FROM tbl_b b JOIN tbl_a a ON b.tbl_a_id = a.id GROUP BY b.tbl_a_id HAVING A.ACTIVE = TRUE");
+    	rs.next();
+        assertContains(rs.getString(1), "/* PUBLIC.TBL_B_IDX: TBL_A_ID = A.ID */");
+        conn.close();
+    }
 }

h2/src/test/org/h2/test/db/TestTableEngines.java

@@ -228,7 +228,7 @@ public class TestTableEngines extends TestBase {
         checkPlan(stat, "select * from t where a > 0 and b > ''", "IDX_B_A");
         checkPlan(stat, "select * from t where b < ''", "IDX_B_A");
         checkPlan(stat, "select * from t where b < '' and c < 1", "IDX_C_B_A");
-        checkPlan(stat, "select * from t where a = 0", "scan");
+        checkPlan(stat, "select * from t where a = 0", "IDX_C_B_A");
         checkPlan(stat, "select * from t where a > 0 order by c, b", "IDX_C_B_A");
         checkPlan(stat, "select * from t where a = 0 and c > 0", "IDX_C_B_A");
         checkPlan(stat, "select * from t where a = 0 and b < 0", "IDX_B_A");
@@ -535,11 +535,11 @@ public class TestTableEngines extends TestBase {
                 + "/* SELECT A FROM PUBLIC.T /++ PUBLIC.\"scan\" ++/ */ "
                 + "INNER JOIN PUBLIC.T /* batched:test PUBLIC.T_IDX_B: B = Z.A */ "
                 + "ON 1=1 WHERE Z.A = T.B");
-        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.\"scan\" */ "
+        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.T_IDX_B */ "
                 + "INNER JOIN ( SELECT A FROM PUBLIC.T ) Z "
                 + "/* batched:view SELECT A FROM PUBLIC.T /++ batched:test PUBLIC.T_IDX_A: A IS ?1 ++/ "
                 + "WHERE A IS ?1: A = T.B */ ON 1=1 WHERE Z.A = T.B");
-        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.\"scan\" */ "
+        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.T_IDX_A */ "
                 + "INNER JOIN ( ((SELECT A FROM PUBLIC.T) UNION ALL (SELECT B FROM PUBLIC.U)) "
                 + "UNION ALL (SELECT B FROM PUBLIC.T) ) Z /* batched:view "
                 + "((SELECT A FROM PUBLIC.T /++ batched:test PUBLIC.T_IDX_A: A IS ?1 ++/ WHERE A IS ?1) "
@@ -548,12 +548,12 @@ public class TestTableEngines extends TestBase {
                 + "UNION ALL "
                 + "(SELECT B FROM PUBLIC.T /++ batched:test PUBLIC.T_IDX_B: B IS ?1 ++/ WHERE B IS ?1)"
                 + ": A = T.A */ ON 1=1 WHERE Z.A = T.A");
-        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.\"scan\" */ "
+        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.T_IDX_A */ "
                 + "INNER JOIN ( SELECT U.A FROM PUBLIC.U INNER JOIN PUBLIC.T ON 1=1 WHERE U.B = T.B ) Z "
                 + "/* batched:view SELECT U.A FROM PUBLIC.U /++ batched:fake PUBLIC.U_IDX_A: A IS ?1 ++/ "
                 + "/++ WHERE U.A IS ?1 ++/ INNER JOIN PUBLIC.T /++ batched:test PUBLIC.T_IDX_B: B = U.B ++/ "
                 + "ON 1=1 WHERE (U.A IS ?1) AND (U.B = T.B): A = T.A */ ON 1=1 WHERE Z.A = T.A");
-        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.\"scan\" */ "
+        checkPlan(stat, "SELECT 1 FROM PUBLIC.T /* PUBLIC.T_IDX_A */ "
                 + "INNER JOIN ( SELECT A FROM PUBLIC.U ) Z /* SELECT A FROM PUBLIC.U "
                 + "/++ PUBLIC.U_IDX_A: A IS ?1 ++/ WHERE A IS ?1: A = T.A */ ON 1=1 WHERE T.A = Z.A");
         checkPlan(stat, "SELECT 1 FROM "
@@ -564,7 +564,7 @@ public class TestTableEngines extends TestBase {
                 + "INNER JOIN PUBLIC.T /* batched:test PUBLIC.T_IDX_A: A = Z.A */ ON 1=1 WHERE T.A = Z.A");
         checkPlan(stat, "SELECT 1 FROM "
                 + "( SELECT U.A FROM PUBLIC.T INNER JOIN PUBLIC.U ON 1=1 WHERE T.B = U.B ) Z "
-                + "/* SELECT U.A FROM PUBLIC.T /++ PUBLIC.\"scan\" ++/ "
+                + "/* SELECT U.A FROM PUBLIC.T /++ PUBLIC.T_IDX_B ++/ "
                 + "INNER JOIN PUBLIC.U /++ PUBLIC.U_IDX_B: B = T.B ++/ "
                 + "ON 1=1 WHERE T.B = U.B */ INNER JOIN PUBLIC.T /* batched:test PUBLIC.T_IDX_A: A = Z.A */ "
                 + "ON 1=1 WHERE Z.A = T.A");
@@ -573,14 +573,14 @@ public class TestTableEngines extends TestBase {
                 + "UNION "
                 + "(SELECT A FROM PUBLIC.U /++ PUBLIC.\"scan\" ++/) */ "
                 + "INNER JOIN PUBLIC.T /* batched:test PUBLIC.T_IDX_A: A = Z.A */ ON 1=1 WHERE Z.A = T.A");
-        checkPlan(stat, "SELECT 1 FROM PUBLIC.U /* PUBLIC.\"scan\" */ "
+        checkPlan(stat, "SELECT 1 FROM PUBLIC.U /* PUBLIC.U_IDX_B */ "
                 + "INNER JOIN ( (SELECT A, B FROM PUBLIC.T) UNION (SELECT B, A FROM PUBLIC.U) ) Z "
                 + "/* batched:view (SELECT A, B FROM PUBLIC.T /++ batched:test PUBLIC.T_IDX_B: B IS ?1 ++/ "
                 + "WHERE B IS ?1) UNION (SELECT B, A FROM PUBLIC.U /++ PUBLIC.U_IDX_A: A IS ?1 ++/ "
                 + "WHERE A IS ?1): B = U.B */ ON 1=1 /* WHERE U.B = Z.B */ "
                 + "INNER JOIN PUBLIC.T /* batched:test PUBLIC.T_IDX_A: A = Z.A */ ON 1=1 "
                 + "WHERE (U.B = Z.B) AND (Z.A = T.A)"); 
-        checkPlan(stat, "SELECT 1 FROM PUBLIC.U /* PUBLIC.\"scan\" */ "
+        checkPlan(stat, "SELECT 1 FROM PUBLIC.U /* PUBLIC.U_IDX_A */ "
                 + "INNER JOIN ( SELECT A, B FROM PUBLIC.U ) Z "
                 + "/* batched:fake SELECT A, B FROM PUBLIC.U /++ PUBLIC.U_IDX_A: A IS ?1 ++/ "
                 + "WHERE A IS ?1: A = U.A */ ON 1=1 /* WHERE U.A = Z.A */ "